1. Field of the Invention
The present invention relates to signal processing, more specifically to a method and system for processing subband signals using adaptive filters.
2. Background
It is well known that a noise cancellation system can be implemented with a fullband adaptive filter working on the entire frequency band of interest. The Least Mean-Square (LMS) algorithm and its variants are often used to adapt the fullband filter with relatively low computation complexity and adequate performance when the interfering signal is white. However, the fullband LMS solution suffers from significantly degraded performance with colored interfering signals due to large eigenvalue spread and slow convergence. Moreover, as the length of the LMS filter is increased, the convergence rate of the LMS algorithm decreases and computational requirements increase. This is problematic in applications, such as acoustic echo cancellation, which demand long adaptive filters to model the return path response and delay. These issues are especially important in portable applications, where processing power must be conserved.
As a result, subband adaptive filters (SAFs) become an interesting and viable option for many adaptive systems. The SAF approach uses a filterbank to split the fullband signal input into a number of frequency bands, each serving as input to an adaptive filter. This subband decomposition greatly reduces the update rate and the length of the adaptive filters resulting in much lower computational complexity.
Subband signals are often maximally decimated in SAF systems by critical sampling. This leads to a whitening of the input signals and an improved convergence behavior. For example, there is an SAF system with critical sampling (A. Gilloire and M. Vetterli, “Adaptive Filtering in Subbands with Critical Sampling: Analysis, Experiments and Applications to Acoustic Echo Cancellation”. IEEE Trans. Signal Processing, vol. SP-40, no. 8, pp. 1862-1875, August 1992).
However, the maximal decimation/critical sampling creates aliasing problems. The presence of aliasing distortion requires the use of adaptive cross-filters between adjacent subbands or gap filterbanks. Systems with cross-filters generally converge slower and have higher computational cost, while gap filterbanks produce significant signal distortion.
It is therefore desirable to provide a method and system for processing subband signals using adaptive filters, facilitating high speed processing, low power consumption and high quality.